#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<limits.h>

int main(void)
{
	int M,N,sum;
	while(scanf("%d%d",&N,&M)!=EOF)
	{
		sum=M*N;
		if(sum%2)
		{
			sum=sum/2+1;
		}
		else
		{
			sum/=2;
		}
		printf("%d\n",sum);
	}
	
	return 0;
}
